1. Field of the Invention
This invention relates to a protective relay system for protecting a power transmission system by using digital data obtained through analog-to-digital conversion of electric quantities in the power transmission system.
2. Description of the Prior Art
Recent trend for greater quantitative scales and complications of power transmission system has been dictating increase of size and complications of construction of protective relay systems for effecting protection and control of such power transmission systems. Such protective relay systems have constructions with their component parts coupled together in very complicated fashions, so that extreme difficulties are involved in their manufacture and maintenance. In order to obviate these difficulties, there have been efforts to reduce the size of and standardize protective relay systems and develop digital protective relay systems, which have resort to programs and other software and and have enhanced versatility of dealing with many protection subjects, by using digital electronic computer techniques.
In such a digital protective relay system, physical quantities representing the status of a power transmission system, for instance voltage, current and other analog electric quantities, are sampled and converted to digital data, which are written in a data memory and used for arithmetic operations carried out according to a relay calculation program stored in a read-only memory (ROM) under the control of a central processor unit (CPU), and the results of calculations are compared with a set value representing a relay operation threshold value set in a setting unit. Where the digital protective relay system is used, for instance, for providing overcurrent protection of a power transmission system, a program for executing calculations on an overcurrent relay operation judgment formula EQU I.sup.2 (=im.sup.2 +i.sub.m-.alpha..sup.2)&gt;K.sub.1.sup.2
is stored in the ROM. In this formula, i.sub.m and i.sub.m-.alpha. are sampled instantaneous current values, and K.sub.1 is an overcurrent relay operation thereshold value stored in the ROM. When the power transmission system current exceeds the preset value set in the setting unit, it is detected as overcurrent, that is, I.sup.2 &gt;K.sub.1.sup.2 is detected. The signal of detection is delivered through an input/output (I/O) device as a trip instruction of a circuit breaker inserted in, for example, a transmission line of the power system, and also sent to an external display unit for display, if desired.
In a case where the digital protective relay system is used for protecting the power transmission system against undervoltage, a determination formula EQU v.sup.2 (=v.sub.m.sup.2 +v.sub.m-.alpha..sup.2)&lt;K.sub.2.sup.2
must be stored in the ROM. In this formula, v.sub.m and v.sub.m-.alpha. sampled instantaneous voltage values, and K.sub.2 is an undervoltage relay operation threshold value set in the setting unit. It will be seen that in order to carry out calculations for a protective action with respect to a different subject, against which the power transmission system is to be protected, it is necessary to change the operation judgment formula to one concerning the different subject, that is, it is necessary to change the relay calculation program stored in the ROM. Usually, the ROM used is an erasable-programmable ROM (EPROM), which is capable of changing its content by infrared beam irradiation, or a fuse type ROM, with which the content once written cannot be changed. In the case of the former ROM, the stored data are erased, and new data are then written by using a writing means. In the latter case, the chip of the ROM is replaced. However, in the former case the stored data have to be tediously changed, and in the latter case a new ROM chip has to be purchased. Particularly, up to data LSI devices in which the data memory, CPU, ROM and so forth are integrated in a single chip are used, and in such a case in order to change the stored program in the ROM the LSI device itself has to be refabricated, and this leads to high cost.